The invention relates to a biological filter for the purification of waste gasses, which filter is provided with a bed of pelletized biologically active compost carrier material, and also to a method for the preparation of the biologically active carrier material.
Such a biological filter for the purification of waste gasses is known. The working of biological filters in general is based on the removal of undesirable components from waste gasses resulting from the decomposition of these components by microorganisms. Hereby one distinguishes between three fundamentally different forms of the process: biofilters, biowashers and trickling filters. In biowashers the microorganisms are freely dispersed in a water phase containing dissolved nutrients, while in trickling filters and biofilters the microorganisms are immobilized on a carrier material arranged in a packed bed. In trickling filters water or a nutrient medium is continuously or intermittently recirculated over a packing. In biofilters there is no recirculation of water, but, in order to maintain the moisture level of the filter material, the necessary water is added from time to time by spraying the packing.
The carrier material in biofilters has three functions. In the first place the carrier material serves to give the packed bed a loose structure so that it can easily be streamed through without much energy loss. The carrier material serves further for the retention of the microorganisms, which take care of the decomposition of the undesirable components from the waste gasses. Finally the carrier material serves as water and nutrient storage for the microorganisms.
The carrier material in biological filters usually consists of particles of a size such that apart from a reasonable specific surface (surface per unit volume) also an acceptable flow resistance is guaranteed. Too small a specific surface results in the filter having a volume that is too large and too uneconomical, while if the filter resistance is too high, too much energy is used when the gas stream is lead through the filter.
The microorganisms that are responsible for the decomposition of the (mainly) organic components, are for their metabolism dependent on the various inorganic nutrients. For this reason carrier materials are used which are able to supply these nutrients (with biofilters: compost, and to a lesser degree peat, heather, tree-bark and the like) or nutrients in solution are continuously or intermittently dispersed over the carrier material (trickling filters).
As carrier materials with the highest density of microorganisms most kinds of compost are applicable. However, these materials (and also peat, heather and tree-bark) have the common disadvantage that they age very quickly. As cause for the carrier material's ageing one could name the decrease of the fibrous structure (lignin- and cellulose fibres) due to biological conversions. Apart from that, moisture loss resulting from too low a relative humidity and/or a rise in temperature in the carrier material has a very negative effect on the packing.
Lignin and cellulose constitute ligno-cellulose-complexes, giving vegetable material (such as the basis for heather, peat and also compost, the latter also consisting of animal material) firmness and flexibility. Lignin consists of a mixture of polymerized phenyl propane-like components, which, in contrast to the fibrous cellulose molecules, are strongly esterified and due to interconnection form very large, complex molecules. Lignin is rigid, giving firmness to the fibres. It is very resistant to enzymatic decomposition; only a small group of microorganisms (fungi that can produce phenyl oxidases: so-called white-rot fungi) are able to (aerobically) break down this type of molecules. Cellulose is much more easy to break down and gives fibres just flexibility. Nevertheless, in biofilters decomposition of lignin takes place also, even though it is a gradual process. The break down of cellulose and lignin fibres results in the material becoming ever finer (more crumbly).
Many kinds of compost have the tendency to form lumps which results in a strong decrease of the specific surface. Lump formation in operational biofilters may possibly be the result of shrinkage of the mycelia (thread-like manner of growth) of fungi and actinomycetes in the compost. In dry conditions the fungi have selectively the advantage over bacteria, so that more mycelium growth occurs. With further dehydration the mycelium threads shrink and cause the compost particles, in which they are anchored, to be drawn together to form a larger aggregate. When this aggregate consists of much crumbly fine material, an almost impermeable solid mass is formed (lump). Sticky/gum-like components in the compost and fungus/actinomycete mycelia help finer particles to bind into lumps.
Due to certain parts solidifying into a lump, other places in the packing develop cracks (shrinkage cracks), causing very undesirable short-circuit currents. Such phenomena are usually irreversible, which means that once a shrinkage crack and lump have developed, they will not disappear on their own account.
Ageing in the carrier material is often revealed by the occurrence of shrinkage cracks in the carrier material. As mentioned above, shrinkage cracks may also develop due to intense drying out of filter material that is not as yet old.
The rise in temperature in the carrier material is inherent to microbiological activity. This activity may, as a result of released oxidation energy, cause the temperature to rise. This implies that biologically active filters have the tendency to dry out.
It is therefore very important that the carrier material retains a sufficiently high water content. It is known that to this end the waste gas, before entering the filter bed, must have as high a relative humidity as possible (&gt;95%) and that moreover, because of the rise in temperature in the material resulting from biological activity, additional moisturizing of the filter bed must regularly take place. The disadvantage is, that due to frequent additional moisturizing--apart from settling of the bed--migration of fine particles occurs, the initially loose structure of the compost deteriorates fast and many open and porous surfaces will silt up. The more the material crumbles, the stronger these effects become: smudging occurs on the material.
A biological filter for the purification of waste gasses, which filter is provided with a bed of pelletized biologically active compost carrier material, is known from the WO-A-92 19357. According to this document compost is pelletized into cylindrical pellets under pressure (20-200 bar) in an apparatus (for instance an extruder) together with up to 50% mineral additives, such as bentonite. The additives serve to provide the necessary firmness.
The objective of the invention is now to provide a biological filter, whose biologically active material contains only compost and can be obtained easily without additives and pressure while still having the required firmness.